During laser cutting, as the laser beam pierces into metal workpieces, bulges of molten and re-solidifying metal or metal oxide occur in the region around the piercing hole wherein the dimensions of the bulges are greatly increased with greater plate thicknesses. These adhesions around the piercing hole create problems mainly when internal geometries have to be cut and with small components in which the cutting contour has to run very close to the piercing location. Also a distance sensing system in the laser machining head detects the bulges as an interfering contour and controls the movement of the laser machining head away from the workpiece, which results in process-related problems. In order to avoid these problems, it is necessary to reduce or prevent bulges adhering to the workpiece.
Japanese patent publication JP11090670A discloses removing, after the completion of the piercing operation, the adhering molten metal by a circular movement around the bulge with the nozzle and blowing the adhering molten metal away with a gas jet at high pressure.
Japanese patent publication JP2009190064A discloses a piercing method wherein, in a first method step, the piercing is done using oxygen as the process gas so that a metal oxide bulge is formed on the surface of the workpiece which does not adhere as strongly as the molten metal. In a second method step, full piercing is done using nitrogen as the process gas, wherein molten metal is deposited on the metal oxide. The bulge is removed from the surface of the workpiece after the piercing operation by blowing off with a blast of gas or by having the nozzle travel across the bulge, i.e. in a scraping movement.